There is a need for a device that will allow intermixing of two small quantities (from about 5 to about 10 .mu.l each) of low relative viscosity liquids, prior to the mixture being metered or dispensed onto or into some other element. An example of such a need is in the field of the immunoassay of biological analytes.
More specifically, recent advances in the analysis of biological liquids such as serum have allowed the use of as little as 10 .mu.l of the liquid, for the test. Examples of test elements suitable for radiometric assays using such small amounts are described in U.S. Pat. Nos. 3,992,158, issued on Nov. 16, 1976, and 4,258,001, issued on Mar. 24, 1981. A large number of analytes have been proven to be amenable to such tests.
One portion of clinical analysis which would benefit from the use of such test elements is the field of immunoassay. Immunoassays usually involve the formation of an antibody-antigen complex in which known amounts of labeled antigen (or antibody) compete with unlabeled, unknown amounts of the same antigen (or antibody) from the patient. In conducting such tests, one procedure adds the labeled antigen or antibody as a diluent to the patient's sample immediately prior to adding the sample to the test element. The total liquid as diluted still preferably is only from about 10 .mu.l to about 20 .mu.l, to minimize the amount of labeled antigen or antibody that is required. Because of the porosity and high rates of flow present in test elements of the types described in the aforesaid U.S. patents, it is difficult in all embodiments to achieve such dilution or mixing by applying first a drop of the sample to such a test element, and then a drop of the diluent. The rapid absorption by the test element in such embodiments prevents the two sequentially added drops from mixing.
However, if the mixing of the diluent and patient sample occur within a container prior to contact with the test element, one or two problems are likely to occur: either the liquids will not intermix well because their small quantities will be spread out onto the walls of the container, or if intermixed, they still will be primarily on the container walls, with very little left to meter. The problem is particularly acute when, as in immunoassays, the diluent and the patient sample occupy about equal volumes, e.g., from about 5 .mu.l of about 10 .mu.l each.
Although conventional metering devices of various kinds have been available prior to this invention, none of them have been adequate to solve the aforementioned problems. For example, very complex devices have been constructed to cause a stream of drops to impinge on each other in mid-air. Such a system is generally unacceptable for clinical analysis metering, due to its complexity and instability.